public class SomeController : Controller
{
    // GET: /SomeIndex/
    [HttpGet]
    public ActionResult Index(int id)
    {
        SomeIndexViewModel vm = new SomeIndexViewModel(id) { myid = id };
        //Do some processing here
        return View(vm);
    }

    // POST: /SomeIndex/
    [HttpPost]
    public ActionResult Index(SomeIndexViewModel vm)
    {
        bool validationsuccess = false;
        //validate
        if (validationsuccess)
            return RedirectToAction("Index", new {id=1234 });
        else
            return View(vm);
        }
    }
}